1. Field of the Invention
This invention relates to surface protective films and, more particularly, to surface protective films that develop strong initial tack at room temperature, that will experience limited changes in adhesive strength with the lapse of time after film attachment and that are sufficiently low in blocking force to provide great ease in the handling or films being rewound.
2. Description of the Related Arts
Metal plates such as aluminum, steel and stainless steel plates, coated metal plates of the same species, and other workpieces such as glass and synthetic resin plates may potentially experience the corrosion, dust deposition or damage problem during transport or storage. With a view to preventing these problems, the surfaces of the workpieces are covered with protective films. The applied surface protective films are stripped from the workpieces at a suitable time either during or after the shaping operation.
There are basic needs that must be met by the surface protective films: they must develop sufficiently strong initial tack to permit easy bonding to the adherend; they should not easily separate from the adherend during its transport or storage; they must be readily stripped from the adherend as required either during or after the working operation. To this end, the surface protective films must satisfy the following requirements: they should have an appropriate degree of adhesiveness to the surface of the adherend to be protected; the films should be soft enough to avoid damaging of the surface to be protected; they should have resistance to corrosion and oils such as machine oils; these properties of the protective films should not change with time or temperature; the films should have mechanical characteristics such as elongation characteristic, scratch resistance, tear resistance, ease in cutting and blanking operations that are appropriate for various methods of working or shaping the adherend; and the films should have high heat resistance. For use in certain applications, the films are required to have satisfactory appearance, transparency and colors in the absence of any film defects such as gelation and fish eyes.
Finally, the surface protective films of the kind contemplated herein which are consumed in large volumes and which are discarded within short life cycles must be produced at an economically feasible low cost. Conventionally, such surface protective films are in most cases produced by coating an acrylic or rubber-based adhesive onto one surface of base films that are chiefly composed of polymers such as low-density polyethylene, polyvinyl chloride and polypropylene. Surface protective films of a dual structure that are produced by co-extrusion of polyethylene and a vinyl acetate resin find use in certain areas and they comprise a polyethylene base layer and a self-adhesive layer made of the vinyl acetate resin.
Further, the surface protective films bonded to the adherend are often processed by cutting, bending, drawing, etc. In order to prevent separation from the adherend during processing, it is common practice to form an adhesive layer of an adhesive that is so formulated as to develop strong initial tack. However, a surface protective film that has an adhesive coated on one surface of the base layer to form an adhesive layer has a problem with the adhesion between the base layer and the adhesive layer and suffers a defect in quality in that the adhesive cannot be completely stripped from the base layer in a later stage and there is a high likelihood for a portion of the adhesive to be left on the base layer. As a further problem, such protective films are more costly than films that have no adhesive layers since their production involves the preparation of a film or the base layer, applying a surface treatment to that base film, coating an adhesive composition that is dissolved in a solvent and then drying the solvent.
The conventional surface protective film that is produced by co-extrusion of two layers has problems. The problem that is worth particular mention is that the carboxylate groups in the vinyl acetate units that compose the self-adhesive layer have bonding property, causing great changes in adhesive strength with the lapse of time; therefore, such surface protective films are only applicable to limited types of resin plates and have a restriction on the field of application. Further, the surface protective film is offered in close winding in the length of several hundreds to several thousands. If the adhesive layer is formed of an adhesive formulation that will develop strong initial tack, the force of blocking between the adhesive layer and the base layer increases so much that the film rewinding operation on bonding to the adherend becomes difficult enough to reduce the ease in handling procedures for bonding.